1. Field of Invention
The present invention relates to a method and structure for intensifying track seeking signals from an optical disk. More particularly, the present invention relates to a method and structure for intensifying track-seeking signals from a recordable digital versatile disk (DVD-R).
2. Description of Related Art
Due to a large storage capacity, digital versatile disk (DVD) has become a favored medium for holding optical data since its introduction in 1997. According to the development history of the recordable compact disk (CD-R), the demand for DVD-R is likely going to be fast too.
However, the production of DVD-R faces one major issue. Fabricating a DVD-R is significantly more difficult than a conventional CD-R. The spiraling track from inner to outer area in a conventional CD-R has a track depth of about 160 nm˜220 nm, a track width of about 500 nm˜800 nm and a track pitch of about 1.6 μm. On the other hand, a conventional DVD-R has a track depth of about 140 nm˜200 nm, a track width of about 300 nm˜500 nm and a track pitch of about 0.74 μm. Both the CD-R and the DVD-R are manufactured in a similar way. First, spiraling tracks are etched onto a stamper mold. Thereafter, injection molding is repeatedly carried out to produce a large number of substrates with the spiraling tracks. Next, a layer of dye having a thickness between 50˜200 nm is formed over the substrate by spin-coating. Finally, a reflection layer and a layer of protective plastic are sequentially formed over the layer of dye.
The spin-coating process in the fabrication of CD-R and DVD-R mainly serves to spread a uniform layer of dyestuff on the substrate. By controlling the coating parameters, the amount of dye settling inside the tracks and on the landing area between the tracks can be precisely adjusted so that optical path difference (Ld) between the track and the landing area is proper. Through careful adjustment of the optical path difference, a disk manufacturer is able to control the intensity of the track seeking signals and influence the recording quality of the disk. Obviously, another method of changing the optical path difference is to modify depth and width of the tracks. Track depth has a direct influence while track width has an indirect influence on the optical path difference. In general, difference in track depth between the DVD-R and CD-R is small but track width of DVD-R is only half that of the CD-R. Hence, coating has a considerable effect on the optical path difference of a DVD-R. This means that the spraying identical dyestuff onto the tracks and landing of a CD-R and DVD-R to form a layer of dye will result in considerable difference in optical path due to a halving of the track width of the DVD-R compared with the CD-R. Proper control of the optical path difference is normally achieved by selecting the right dye, selecting a suitable solvent, adjusting the track depth and width and controlling the coating parameters. Controlling the coating parameters means adjusting the dye concentration within the coating solvent, the coating speed, the rotation period and so on. Although there seems to be a host of parameters that can be selected to provide proper adjustment, the number of alternatives is quite limited in practice. The substrate of a disk is normally fabricated using polycarbonate material. Since polycarbonate material resists most solvents poorly, the permissible options are only alcoholic and petroleum solvents. However, most alcoholic or petroleum solvents are poor solvents for dissolving dyes leading to a severe limitation on the type of dyes that can be selected. Furthermore, too deep a track is difficult to fabricate by injection molding. In general, a CD-R having a track depth approaching 200 nm is hard to make. Therefore, the fabrication of DVD-R with such a track depth is even harder because a DVD-R has a track width only half that of the CD-R. In other words, possible variation in the spatial dimension for a DVD-R is quite limited. Finally, the effect caused by tuning the coating parameters is only minor.